Fuel Injection - An Introduction to Siamese Problems

(With thanks to Keith Clarke for inspiration and information)

Introduction

What is electronic fuel injection, or EFI?  I guess that it has been around long enough for most to know that it is simply injectors that squirt the fuel into the inlet manifold rather than using carburettors.  Where it has advantages over standard carburettors is the ability for it to take information from the mixture via a sensor in the exhaust, along with ambient air temperature, coolant temperature, and information from the manifold absolute pressure (MAP) and throttle position sensor (TPS).  This information can be used by the engine management to then tell the injectors when and how much fuel to squirt.

With the addition of EDIS and the crank position sensor and TPS on my BGT, I started thinking about the possible next step up to Fuel Injection, and so this page is really dedicated to the issues of fitting injection to a B series.

Whilst I believe that a properly set up pair of SU carbs do an excellent job, and according to the experts, still have more fuel mapping points than an injector can ever have, the engineering mind will always want to review the potential for new ideas.  In addition to which, with the ever increasing legislation and environmental issues, it seems to make sense to explore avenues that may help to keep the B on the road.  That said, currently in the UK, tax-exempt cars only have a visual emissions check, but post 1978 cars are required to be tested, though the limits are nowhere near as stringent as modern cars.  However, in places like North America some States have such tight limits that Moss have started to offer a fuel injection unit for owners (see below).

Photo from Moss web site

As with the EDIS, the often quoted benefits of the EFI are better cold starting and better fuel economy, but not necessarily more power.  Moreover, whilst the SU's don't necessarily need re-setting all that often, the EFI will stay in tune and can adapt to varying conditions accordingly.  EFI really became a necessity with the advent of the catalytic converter on exhaust systems, but even in the 1960's Triumph were using a form of mechanical injection which was quite advanced for its time.  According to Roger Parker, even in 1979 the plan was to offer the B with the O Series engine in both twin SU and EFI for markets with stricter emission.

A basic review of EFI is that the tank will normally have a high pressure pump feeding a fuel rail, and excess fuel is fed back to the tank via a return.  As mentioned above, input to the decision making in the ECU come from various sensors, which then squirt accordingly.  It's a feedback looping process as the Oxygen sensor in the exhaust determines how well the fuel is burning.

So how easy is it to fit to the B Series?

Not really possible is the short answer!  To understand EFI in more detail I suggest you visit Keith Clarke's excellent site.  Admittedly, Keith's site is dedicated not to the B but to the C, however, his research and detail is well worth spending time reading.

To summarise an email Keith kindly took the time to compose for me:

The C has individual inlet ports so it doesn’t suffer the charge-robbing issues of the A & B-series engines.

On the siamese-ported engines, there have been some experiments with port injection (up near the head, so the manifold remains ‘dry’ only delivering air). The only hope for charge-robbing is sequential injection (expensive ECU that manages one squirt per cylinder, per induction stroke) but this requires large injectors dumping lots of fuel quickly (whilst only the correct inlet valve is open) & as engine speed increases this becomes impossible. This approach hasn’t been too successful – atomisation suffers as well as part throttle drivability, as big injectors have poor sensitivity when they are opening for very short durations at part-throttle. Most A-series & B-series converters therefore accept the limitations of siamesed ports & go for an aggregated, wet manifold approach.

So for it to work on a B series with Siamesed inlets, then, we need to either look at the way the system works in a Mini, or fit an expensive cross-flow head!  A note here on the Mini, back in 1993, when catalytic converters became mandatory in the UK, the A+ Series engine in the Mini was fitted with single-point fuel injection.  However, for a serious update in 1996 that included many other new features, the cars were fitted with twin-point fuel injection.  This required some clever programming to get it to overcome the issues related to the charge-robbing effect, a solution that was patented by Rover.  The diagram below shows the inlet system used on a Siamesed arrangement such as the A series.

In fact, from the actual patent document, the problem of charge robbing is very well described:

"Use of single point injection or a carburettor results in uneven supply of fuel to the cylinders.  In a four cylinder engine with a standard 1-2-3-4 firing sequence, the inlet valve for cylinder 2 opens and draws fuel from the manifold and then the inlet valve for cylinder 1 opens before the fuel in the manifold is fully replenished.  Cylinder 2 (sic - I think should read as Cylinder 1) therefore does not produce as much power as cylinder 1 (see my other comment, this should be 2).  The same inequality occurs between cylinders 3 & 4."

This is overcome as described below in the patent, and in the diagram above (Fig. 4) where the shaded areas marked O and C represent the inlet valve opening and closing during a complete cycle of the engine.

For a given cycle based rather on 2-1-3-4 so as to better follow this, injector 1 feeds cylinders 2 and 1 either as two discreet bursts (solid lines) or as one continuous burst (dotted lines).

The twin-point injection Minis were a big step up from the single point injection systems, although it should be noted that in the late 1980s, single point injection was quite common on many engines even with individual inlets, even on the K Series engines.  Nowadays most run an injector per inlet.

See fitting EFI to a B series.

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